Method and system for managing power delivery for power over Ethernet systems

ABSTRACT

A method for managing the delivery of power for a plurality of devices includes allocating a power limit for each of the plurality of devices and providing power to at least one of the plurality of devices. The method also includes, in response to the providing of power, measuring the amount of power utilized by each of at least one of the at least one of the plurality of devices and determining that the amount of power exceeds the power limit for the device.

TECHNICAL FIELD OF THE INVENTION

This invention relates generally to power supply systems and moreparticularly to a method and system for managing the delivery of powerfor power over Ethernet (PoE) systems.

BACKGROUND

Power over Ethernet systems are seeing increasing use in today'ssociety. Power over Ethernet, sometimes abbreviated PoE, refers toproviding power to Ethernet devices over an Ethernet line that is alsoused to communicate data. Thus, power over Ethernet devices do notrequire separate power supply lines. In some instances, the power may besupplied by a power supply contained within an Ethernet switch. Becausethe power supply does not generally have the power capability to supplymaximum power to every port, there is a limit on the number of powerover Ethernet devices that can be connected to a given power supply. Aport may be denied power, if it will result in oversubscription of thepower supply. Example power over Ethernet devices that can benefit fromreceiving power over the Ethernet communication lines include aninternet protocol telephone, a badge reader, a wireless access point, avideo camera, and others.

Traditionally, when a power over Ethernet device is connected to a powersupply, the power over Ethernet device is allocated a maximum powerclass according to IEEE standard 802.3af denoted as class 0 thru 4.These maximum values correspond to the maximum amount of power that willbe supplied by the power supply to the power over Ethernet device. IEEEstandard 802.3af provides for three levels of 15.4 watts, 7.5 watts, and4.0 watts for these power over Ethernet devices. In certaincircumstances, such allocation prevents the power supply from beingutilized to its full capability due to the coarse granularity in class.

A software program referred to as Cisco Discovery Protocol allows formore granular specification of the limit for the power over Ethernetpowered devices other than the above-described IEEE levels. However, thepower supply still may have unutilized capacity.

SUMMARY OF THE INVENTION

A method for managing the power provided for a plurality of devicesincludes allocating a power limit for each of the plurality of devicesand providing power to at least one of the plurality of devices. Themethod also includes, in response to the providing of power, determiningthe amount of power utilized by each of at least one of the at least oneof the plurality of devices and determining that the amount of powerexceeds the power limit for the device.

Embodiments of the invention provide numerous technical advantages.Some, none, or all of the embodiments may benefit from the followingadvantages. According to one embodiment of the invention, powereddevices in a power over Ethernet system may be oversubscribed because auser specified power limit for the device may be utilized in combinationwith monitoring of the power utilized by that device and adjustment ofits limit. Thus, where the power supply is not being utilized to itsfull capacity, the power limit for a particular device may be increasedto allow for additional power requirements of that device. This allowsgreater use of the full capability of a power supply. According toanother embodiment, greater usage of a power supply may be achieved byoversubscribing a power supply in conjunction with measurement of theoverall usage of the power supplied by the power supply. If the overallusage exceeds a specified level, power may be terminated to one or moreof the power devices until acceptable usage levels are met.

Other advantages may be readily apparent to those skilled in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference is now made to the following description taken in conjunctionwith the accompanying drawings, wherein like reference numbers representlike parts, in which:

FIG. 1 is a block diagram of portions of a power over Ethernet systemshowing components of the power supply and connections to the powereddevices;

FIG. 2 is a block diagram of an example of the control system of FIG. 1;

FIG. 3 is a flowchart illustrating example steps associated withmanaging the provisioning of power to a plurality of powered devicesaccording to the teachings of one embodiment of the invention;

FIG. 4 is a block diagram of an alternative embodiment of a power supplyaccording to the teachings of the invention;

FIG. 5A is a flowchart illustrating example steps associated with theprovision of power to a plurality of powered devices according to theteachings of another embodiment of the invention; and

FIG. 5B is a flowchart illustrating example steps associated with thestep in FIG. 5A of identifying a problem with providing power to one ofthe powered devices.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

Embodiments of the invention are best understood by referring to FIGS. 1through 5B of the drawings, like numerals being used for like andcorresponding parts of the various drawings.

FIG. 1 is a block diagram of a system 10 associated with the provisionof power to a plurality of power over Ethernet devices. System 10includes power over Ethernet power source equipment 12, which providespower to a plurality of power over Ethernet powered devices 22.Associated with power over Ethernet power source equipment 12 is acontrol system 14. Control system 14 manages the provisioning of powerby power over Ethernet power source equipment 12 to powered devices 22.Power over Ethernet power source equipment 12 includes a power supply 16associated with a plurality of ports 18. In this embodiment, each port18 has an associated programmable power monitor 20 that monitors thepower supplied over that port to the respective power device 22. Controlsystem 14 may increase the user specified power limit for each device inresponse to a determination that the particular device is utilizing morepower than its specified limit. Control system 14 also compares the sumsof the power supplied to the plurality of powered devices 22 against thecapacity of the power supply 16, to determine if there exists additionalcapacity to provide more power to powered devices 22 or if the systemlimit is exceeded. This allows oversubscription of power supply 16. Alsoillustrated in FIG. 1 is a user interface 15.

User interface 15 allows a user to specify a power limit for each powerover Ethernet powered device 22. This power limit specifies the maximumamount of power that power over Ethernet powered device 22 shouldreceive. However, it may be the case that any given power over Ethernetpowered device 22 may actually require more power than its user definedmaximum limit. User interface 15 may be implemented in software orhardware and allows a user to enter power limits for powered devices 22.This allows a user to specify the expected power use of a particularpowered device 22 which may or may not fall near conventional IEEElevels. By allowing user specification of the expected power usage ofeach of the plurality of powered devices, oversubscription for powersupply 16 may occur. This advantage allows for the maximum utilizationof power supply 16. In some embodiments, power over Ethernet powersource equipment 12 may include an overall power monitor that measuresthe overall power supplied by power supply 16 at any given time topowered devices 22.

Control system 14 may be implemented in software or hardware. Oneexample of a software embodiment is described in greater detail inconjunction with FIG. 2. Control system 14 allows the configuring ofpower limits for each device by a user, in conjunction with userinterface 15. This configured value may differ from the conventionalIEEE values of 15.4 watts, 7.5 watts, or 4.0 watts. This provides oneadvantage of allowing more granular control of the power limits forparticular devices, particularly devices whose maximum powerrequirements do not fall near one of the three IEEE levels.

Power supply 16 provides power for the plurality of ports 18 and then tothe plurality of power over Ethernet powered devices 22. Although anysuitable power supply may be used, one example is a 370 watt, 50V,isolated power supply manufactured by Delta Electronics. Althoughillustrated as a single power supply, power over Ethernet power supplyequipment may incorporate multiple power supplies 16.

Ports 18 correspond in a one-to-one fashion to the respective ones ofthe plurality of power over Ethernet powered devices 22. One example ofports 18 is conventionally known in the industry and is therefore notdescribed here in greater detail. However, it should be emphasized thatunconventional ports 18 may also be utilized without departing from thescope of the present invention.

Programmable power monitors 20 measure the amount of power beingutilized and supplied to respective ones of power over Ethernet powereddevices 22. Programmable power monitors 20 may be implemented byhardware or software but in one particular embodiment utilitieshardware. Programmable power monitors 20 may store the power limitassociated with the respective powered device 22; however, the limit maybe stored in other locations, including within control system 14. Themeasurement portion of programmable power monitor 20, according to oneexample, may be formed according to conventional techniques, which arewell known and is not described here for simplicity of description.However, it is noted that programmable power monitor may measure powerutilized by the associated powered device 22 by making both current andvoltage measurements, or by making current measurements only andassuming a known power supply 16 output voltage. Unconventional powermeasurement techniques may also be used to the power measurement portionof programmable power monitor 20.

Powered devices 22 may be any power over Ethernet device that requirespower. Examples include an internet protocol phone, a wireless accesspoint, a video camera, and a badge reader.

In operation, power supply 16 provides power through ports 18 to one ormore of power devices 22. Programmable power monitors 20 measure theamount of power supplied to the respective ones of power over Ethernetpowered devices 22. If it is determined that a particular one of thepower over Ethernet devices 22 exceeds its specified power limit,programmable power monitor communicates with control system 14 overlines 21 informing control system 14 that a particular power overEthernet device is exceeding its power limit. Control system 14 alsoreceives the amount of power being drawn by each power device fromprogrammable power monitors 20. In response to the determination that aparticular power over Ethernet device 22 is exceeding its power limit,control system 14 determines whether power supply 16 is exceeding itsmaximum power limit or whether additional power capability exists. Ifadditional power capability exists, then control system 14 may modifythe power limit associated with a particular power over Ethernet powereddevice 22, allowing that device to receive additional power withoutcreating a fault condition. Control system 14 may make thisdetermination that power supply 16 is operating within its overall powerlimit by adding the power usage of each of the power over Ethernetdevices 22, or alternatively, through use of an overall power monitor,as described above. In one embodiment, in response to control system 14determining that the capacity of the power supply is exceeded, controlsystem 14 removes power from at least one of at least one of the powerover Ethernet powered devices 22. The determination of which ones toremove power from is according to the configuration of control system14, which may be modified by user interface 15.

In this manner, system 10 can be configured with nominal values for eachof the plurality of power over Ethernet devices but adjustments may bemade that represent the actual use. In this manner such a system can besafely oversubscribed.

FIG. 2 is a block diagram of one example of control system 14. In thisexample control system 14 is implemented through use of a softwareprogram. In this example, control system 14 includes a processor 24,memory 26, and storage 28. Memory 26 may store a control application 30as well as power limits 32. The operation of control application 30 isdescribed in greater detail below in conjunction with FIG. 3; however,it should be emphasized that, although the method of FIG. 3 is describedin the context of control application 30, such a method could beimplemented by other means such as through hardware. Storage 28 providesadditional storage that may be utilized by processor 24 in executingcontrol application 30. Processor 24 may take any suitable formincluding custom processors or off-the-shelf processors. It is alsoemphasized that the entire control system 14 also may be implemented inhardware, such as through an application-specific integrated circuit.

FIG. 3 is a flowchart illustrating example steps associated withproviding power to a plurality of Ethernet devices according to theteachings of the invention. In one example, unless the context indicatesotherwise, these steps may be performed by control system 14; however,they may also be performed by suitable hardware or software such as byprogrammable power monitors 20 having suitable logic for implementingthese described steps. The method 50 begins at step 52. At step 54 apower limit is allocated for each power over Ethernet powered device 22.This may be performed automatically by control system 14, by a userthrough user interface 15, or through other suitable methods. Accordingto the teachings of the invention, the allocated power limit for eachdevice may include values other than those specified by IEEE, which are15.4 watts, 7.5 watts, and 4.0 watts; however, these power limits mayalso be utilized.

After allocation of a power limit for each device, power is provided toany device requiring power at step 56. The power limit allocated foreach device may be stored in control system 14 or may also be stored inprogrammable power monitors 20, or in other suitable locations. At step58, power usage for each device is measured. In the example of FIG. 1,such power usage is measured by programmable powered monitors 20.

At step 60 a determination is made of whether the power usage for anypowered Ethernet device exceeds the limit for that device. If not,processing continues at step 58 in which power usage is continuallymonitored for each of the power over Ethernet devices. However, if atstep 60 it is determined that the power usage for any particular deviceis exceeded, then it is determined whether the overall power limit forpower supply 16 is exceeded at step 64. This determination may be basedupon an overall power usage determination that occurs independently ofthis method or may involve specifically determining the power usage forthis purpose, as indicated at step 62. Such determination of an overallpower usage may involve measuring the overall power usage or measuringpower usage for each power over Ethernet device 22 and summing thatusage.

If the overall usage is exceeded, then power may be terminated to thedevice 22 that exceeds its particular limit, as noted at step 66. Inthat event, control system 14 may remove a fault condition that resultedfrom the power over Ethernet device 22 exceeding its power limit.Additionally, a user may be informed that power has been discontinued tothe power over Ethernet device 22. To allow the particular power overEthernet device 22 to resume operation, its power limit may bereconfigured before power is supplied to that device again.

The result of step 64 indicates that the overall limit for the powersupply 16 is not exceeded, then the power limit for the particular powerover Ethernet device 22 may be increased, allowing that device toreceive more power than it was originally allocated. As an alternative,control system 14 may be configured to not allow a preset power limit tobe increased in which case processing would proceed as described abovewith respect to step 66. However, if the power limit is increased atstep 68, the power limit may be increased to any suitable amountincluding those specified by the IEEE 802.3af standard or other values.Processing continues after step 66 and step 68 at step 58 in which powerusage for each device is continually monitored.

FIG. 4 is a block diagram illustrating an alternative embodiment of asystem 110 for providing power for a plurality of power over Ethernetdevices. System 110 is similar to system 10 with like components havinganalogous reference numerals. However, power over Ethernet power supplyequipment 112 includes an overall power monitor 117 that measures theoverall amount of power supplied by power supply 116 to the plurality ofpower over Ethernet devices 122. Power monitor 117 may utilize anyappropriate method of measuring power delivered by power supply 116.This method may include both a current and a voltage measurement or acurrent measurement only, with an known power supply 116 output voltage.Power supply equipment 112 does not necessarily measure the power usagefor each individual powered device. In addition, control system 114 mayexecute different procedures than those described above with respect toFIGS. 1 through 3 in controlling the power supply to a particular powerover Ethernet device 122.

Control system 114 may still receive user specified power limits foreach of the plurality of power Ethernet devices 122 through userinterface 115 or through other suitable system. However, in thisexample, the power limit associated with any particular power device 122is not modifiable in response to a real-time measurement of the amountof power utilized by a particular power over Ethernet device. Rather, inresponse to a determination that the overall power usage by power overEthernet devices 122 exceeds the limit for power supply 116, thenappropriate steps are taken to lower this power usage. Example stepsassociated with controlling power to power over Ethernet devices 122 aredescribed in conjunction with FIGS. 5A and 5B.

FIG. 5A is a flowchart illustrating example steps associated withmanaging power supplied to a plurality of power over Ethernet devices.The method 150 begins at step 152. At step 154 a power limit isallocated for each of a plurality of Ethernet devices. These powerlimits may differ from the three values currently available under IEEEstandard, which are 15.4 watts, 7.5 watts, and 4.0 watts; however, theIEEE values may be used. At step 156 power is provided to any of theplurality over Ethernet devices 122 that require power. At step 158 thecollective amount of power utilized by the power over Ethernet devices122 is determined. In one example, this determination is made by a powermonitor, such as power monitor 117.

At step 168 a determination is made of whether an overall power usagelimit is exceeded. If not, processing loops back to step 158 in whichthe amount of power being utilized is continually monitored. If theoverall usage limit is exceeded, processing continues at step 162 inwhich an attempt is made to identify a particular power over Ethernetdevice that is causing the overall power limit to be exceeded. Theapproach utilized to determine the particular power over Ethernet devicethat is causing the problem may vary. One example is described below inconjunction with FIG. 5B.

As an alternative to step 162 of identifying a particular power overEthernet device that is causing the overall power usage to be exceeded,power to particular devices may be terminated based upon a devicepriority. This priority may specify which power over Ethernet deviceswould receive priority over others in the case where the power supplycannot supply power to all of the devices. This priority may be setthrough use of an interface such as user interface 115, or through othertechniques. As another alternative, power may be terminated to powerover Ethernet devices without attempting to identify the cause of theover-usage problem until the overall power limit is not exceeded,without attention to any priority allocation.

In the embodiment in which the particular power over Ethernet devicecausing the power over-usage problem is identified, processing continuesat step 164 in which power is terminated to the identified device.Processing then returns to step 158 in which the overall power usage iscontinually monitored.

FIG. 5B is a flowchart illustrating one example of steps associated withstep 162 of FIG. 5A of identifying a problem device. In general, method162 involves stepping through each port, terminating power to that port,and observing the effect on the overall power usage. If the overalleffect is a significant decrease in the power usage, this may indicatethe particular port chosen corresponds to the power over Ethernet devicecausing the power usage to be exceeded. In such a case, power isterminated to that device.

More specifically, the method begins at step 170. Each port is steppedthrough with the below-described steps until the port corresponding tothe power over Ethernet device causing the over-usage problem isidentified. At step 174 it is determined whether the port is allocated adefault value, corresponding to one of the IEEE values, or whether theport is allocated a user-specified value. If the port has a defaultvalue corresponding to one of the IEEE values then it is not likely thecause of an over-usage problem, which indicated a step 176, andprocessing returns at step 174 for the next port. However, if the portis not allocated a default value, or all remaining ports are allocateddefault values, then this indicates the port is potentially associatedwith the over-usage problem and power is temporarily removed from thatport, as indicated at step 178. At step 180 a determination is madewhether there is a significant change in the overall power usage. If so,this indicates that the power over Ethernet device associated with thatport is likely the problem and power is temporarily terminated to thatdevice, as indicated at step 182. However, if at step 178 no significantchange in the overall usage is measured, this indicates that this portis not likely the problem and processing continues at the next port backat step 172. It is noted that when referring to a significant change, itis meant to refer to a change in overall power usage greater than theallocated power limit for that particular device. Step 184 may alsoinvolve returning power to the Ethernet device for which power wastemporarily terminated; however, in one example, power remainsterminated until the port causing the problem is identified. The methodconcludes at step 186.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions, andalterations can be made therein without departing from the spirit andscope of the invention as defined by the appended claims.

1. A system for providing power to a plurality of Ethernet devicescomprising: a power supply to provide power to the plurality of Ethernetdevices; a power monitor to determine the amount of power collectivelyutilized by the plurality of Ethernet devices; a power limit system tostore a power limit associated with the respective ones of the pluralityof Ethernet devices; and a control system to terminate power to each ofthe plurality of Ethernet devices in response to a determination thatthe amount of power collectively used by the plurality of Ethernetdevices exceeds a limit for the collective power usage and to furtherdetermine the identity of a particular one of the plurality of devicesthat causes the collective power limit to be exceeded; wherein thecontrol system further determines the identity of a particular one ofthe plurality of devices that causes the collective power limit to beexceeded and terminates power to the particular one of the plurality ofdevices; and wherein the control system further determines the identityof the particular one of the plurality of devices by: terminating powerto one of the plurality of Ethernet devices; measuring the amount ofpower collectively used by the plurality of Ethernet devices with powertemporarily terminated to the one of the plurality of Ethernet devices,and subsequently subtracting the measured amount from the amount ofpower collectively used by the plurality of Ethernet devices prior toterminating power to the one of the plurality of Ethernet devices,thereby calculating a change in overall power usage attributable to theone of the plurality of Ethernet devices; if the change in overall powerusage attributable to the one of the plurality of Ethernet devices isgreater than the power limit associated with the one of the plurality ofEthernet devices, then identifying that the one of the plurality ofEthernet devices is the particular one of the plurality of devices thatcauses the collective power limit to be exceeded, otherwise identifyingthat the one of the plurality of Ethernet devices is not the particularone of the plurality of devices that causes the collective power limitto be exceeded; and re-applying power to the one of the plurality ofEthernet devices and proceeding to temporarily terminate power toanother one of the plurality of Ethernet devices, if the one of theplurality of Ethernet devices is not the particular one of the pluralityof devices that causes the collective power limit to be exceeded.
 2. Asystem as in claim 1 wherein: the power monitor is to determine theamount of power collectively utilized by the plurality of Ethernetdevices by measuring both the current and the voltage; and the controlsystem, when measuring the amount of power collectively used by theplurality of Ethernet devices with power temporarily terminated to theone of the plurality of Ethernet devices, is to measure the amount ofpower by using the power monitor to measure both the current and thevoltage and multiplying the current by the voltage.
 3. A system as inclaim 1 wherein: the power monitor is to determine the amount of powercollectively utilized by the plurality of Ethernet devices by measuringthe current and assuming a constant pre-defined voltage; and the controlsystem, when measuring the amount of power collectively used by theplurality of Ethernet devices with power temporarily terminated to theone of the plurality of Ethernet devices, is to measure the amount ofpower by using the power monitor to measure current and multiplying thecurrent by the constant pre-defined voltage.
 4. A system as in claim 1wherein the control system, when is to determine the identity of theparticular one of the plurality of devices, is to refrain fromterminating power to the one of the plurality of devices if the one ofthe plurality of Ethernet devices has, as its associated power limit oneof a plurality of pre-defined default values.
 5. A system as in claim 4wherein the plurality of pre-defined values are 15.4 watts, 7.5 watts,and 4.0 watts.
 6. A system as in claim 1 wherein the collective powerlimit is a pre-defined maximum amount of power that the power supply isarranged to provide to the plurality of Ethernet devices.
 7. A methodfor providing power to a plurality of Ethernet devices comprising:allocating a power limit for each of the plurality of Ethernet devices,an allocated power limit for at least one of the plurality of Ethernetdevices being other than 15.4 watts, 7.5 watts, and 4.0 watts; providingpower to one or more of the plurality of Ethernet devices; determiningthe amount of power collectively utilized by the plurality of devicesand determining that the amount of power exceeds a collective powerlimit for the plurality of Ethernet devices; and in response todetermining that the collective power limit is exceeded, terminatingpower to at least one, but not all, of the plurality of Ethernetdevices; wherein the particular one of the plurality of devices to whichpower is terminated is a device that is determined to cause thecollective power limit to be exceeded; wherein the method furthercomprises determining the identity of the particular one of theplurality of devices that cause the collective power limit to beexceeded; wherein determining the identity comprises: terminating powerto one of the plurality of Ethernet devices; measuring the amount ofpower collectively used by the plurality of Ethernet devices with powertemporarily terminated to the one of the plurality of Ethernet devices,and subsequently subtracting the measured amount from the amount ofpower collectively used by the plurality of Ethernet devices prior toterminating power to the one of the plurality of Ethernet devices,thereby calculating a change in overall power usage attributable to theone of the plurality of Ethernet devices; if the change in overall powerusage attributable to the one of the plurality of Ethernet devices isgreater than the power limit associated with the one of the plurality ofEthernet devices, then identifying that the one of the plurality ofEthernet devices is the particular one of the plurality of devices thatcauses the collective power limit to be exceeded, otherwise identifyingthat the one of the plurality of Ethernet devices is not the particularone of the plurality of devices that causes the collective power limitto be exceeded; and re-applying power to the one of the plurality ofEthernet devices and proceeding to temporarily terminate power toanother one of the plurality of Ethernet devices, if the one of theplurality of Ethernet devices is not the particular one of the pluralityof devices that causes the collective power limit to be exceeded.
 8. Amethod as in claim 7 wherein: determining the amount of powercollectively utilized by the plurality of devices includes measuringboth the current and the voltage and multiplying the current by thevoltage; and measuring the amount of power collectively used by theplurality of Ethernet devices with power temporarily terminated to theone of the plurality of Ethernet devices includes measuring both thecurrent and the voltage and multiplying the current by the voltage.
 9. Amethod as in claim 7 wherein: determining the amount of powercollectively utilized by the plurality of devices includes measuring thecurrent and the voltage and multiplying the current by a constantpre-defined voltage; and measuring the amount of power collectively usedby the plurality of Ethernet devices with power temporarily terminatedto the one of the plurality of Ethernet devices includes measuring thecurrent and multiplying the current by the constant pre-defined voltage.10. A method as in claim 7 wherein determining the identity furthercomprises: refraining from terminating power to the one of the pluralityof Ethernet devices if the one of the plurality of Ethernet devices has,as its associated power limit one of a plurality of pre-defined defaultvalues.
 11. A method as in claim 10 wherein the plurality of pre-definedvalues are 15.4 watts, 7.5 watts, and 4.0 watts.
 12. A system forproviding power to a plurality of Ethernet devices comprising: a powersupply to provide power to the plurality of Ethernet devices; a powermonitor to determine an amount of power collectively utilized by theplurality of Ethernet devices; a power limit system to store a powerlimit associated with each of the respective ones of the plurality ofEthernet devices; and a control system to determine the identity of aparticular one of the plurality of devices that causes the amount ofpower collectively used by the plurality of Ethernet devices to exceed acollective power limit and terminate power to the particular one of theplurality of devices; and wherein the control system determines theidentity of the particular one of the plurality of devices by:terminating power to one of the plurality of Ethernet devices; measuringthe amount of power collectively used by the plurality of Ethernetdevices with power temporarily terminated to the one of the plurality ofEthernet devices, and subsequently subtracting the measured amount fromthe amount of power collectively used by the plurality of Ethernetdevices prior to terminating power to the one of the plurality ofEthernet devices, thereby calculating a change in overall power usageattributable to the one of the plurality of Ethernet devices; if thechange in overall power usage attributable to the one of the pluralityof Ethernet devices is greater than the power limit associated with theone of the plurality of Ethernet devices, then identifying that the oneof the plurality of Ethernet devices is the particular one of theplurality of devices that causes the collective power limit to beexceeded, otherwise identifying that the one of the plurality ofEthernet devices is not the particular one of the plurality of devicesthat causes the collective power limit to be exceeded; and re-applyingpower to the one of the plurality of Ethernet devices and proceeding totemporarily terminate power to another one of the plurality of Ethernetdevices, if the one of the plurality of Ethernet devices is not theparticular one of the plurality of devices that causes the collectivepower limit to be exceeded.
 13. A system as in claim 12 wherein: thepower monitor is to determine the amount of power collectively utilizedby the plurality of Ethernet devices by measuring both the current andthe voltage; and the control system, when measuring the amount of powercollectively used by the plurality of Ethernet devices with powertemporarily terminated to the one of the plurality of Ethernet devices,is to measure the amount of power by using the power monitor to measureboth the current and the voltage and multiplying the current by thevoltage.
 14. A system as in claim 12 wherein: the power monitor is todetermine the amount of power collectively utilized by the plurality ofEthernet devices by measuring the current and assuming a constantpre-defined voltage; and the control system, when measuring the amountof power collectively used by the plurality of Ethernet devices withpower temporarily terminated to the one of the plurality of Ethernetdevices, is to measure the amount of power by using the power monitor tomeasure current and multiplying the current by the constant pre-definedvoltage.
 15. A system as in claim 12 wherein the control system, when isto determine the identity of the particular one of the plurality ofdevices, is to refrain from terminating power to the one of theplurality of devices if the one of the plurality of Ethernet deviceshas, as its associated power limit one of a plurality of pre-defineddefault values.
 16. A system as in claim 15 wherein the plurality ofpre-defined values are 15.4 watts, 7.5 watts, and 4.0 watts.
 17. Asystem as in claim 12 wherein the collective power limit is apre-defined maximum amount of power that the power supply is arranged toprovide to the plurality of Ethernet devices.